The present invention relates to a method for reducing or even preventing resonance-frequency mechanical oscillations of a wind power plant.
The popularity of wind power plants as sources of renewable energy has in the past few years increased partly due to the fact that the required technology has become less expensive and partly because the energy prices of non-renewable energy sources have increased. At the same time, the unit sizes of wind power plants have been increased, and today the largest working wind power plants are large enough to produce several megawatts of power.
With an increase in nominal electric output, the physical size and mass of the equipment in a wind power plant also increase, which means that the tower supporting the wind turbine, generator, and their peripheral equipment also needs to be strong. In addition, a high-power wind power plant requires a long-blade turbine, in which case the tower also needs to be quite high. The length of the turbine blades in turn directly restricts the rotation speed of the turbine, because the speed of the blade end cannot be allowed to grow arbitrarily already because of the noise it causes.
All the above factors bring about a quite low mechanical resonance frequency of a wind turbine structure. This resonance frequency is typically at a range that it is triggered by the rotation of the turbine. If the turbine rotates for a long time on a frequency that corresponds to the mechanical resonance frequency of the wind turbine structure, there is a danger that the mechanics of the wind power plant can weaken or even fail, thus causing considerable costs. In addition, the noise caused by the resonance situation may disturb the surroundings of the wind power plant.
U.S. Pat. No. 6,525,518 presents a solution with which the start of mechanical resistance of a wind power plant can be detected and the oscillations caused by the resonance reduced. Detecting the resonance is based on a spectrum analysis made on measured quantities. The oscillations are, in turn, reduced by either altering the rotation speed of the rotor or alternatively altering the dynamic load of the turbine. The solutions of U.S. Pat. No. 6,525,518 eliminate resonance situations, but going into resonance is, however, not actively prevented. Thus, the power plant may fall back to the resonance situation. In the solution of U.S. Pat. No. 6,525,518, both alternative ways of reducing the oscillation caused by resonance reduce the power output produced by the wind power plant.